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First, led by a British-speaking murderer, ISIS released beheading videos of US journalists. Then ISIS releases video of a British journalist hostage 'news' report to expose "the truth," and now, Australian police have raided hundreds of homes foiling a direct ISIS provocation to publicly behead a random citizen in Sydney. As Reuters reports, more than 800 police were involved in the pre-dawn security operation in Sydney and Brisbane, which was described as the largest in Australian history. A 22-year-old Sydney man was arrest, after links were found to top ISIS recruiter Mohammad Baryalei that PM Abbot stated were "clearly designed to shock and horrify, perhaps terrify" the community.

Top terrorist recruiter Mohammad Baryalei is behind an Islamic State plot to murder Australians on video, police say, sparking the nation's biggest ever counter-terrorism raids and prompting warnings of a high alert for a further 18 months.

NSW and federal police as well as ASIO officers raided homes across Sydney and Brisbane on Thursday morning, arresting 15 men, some of them as young as 18 or 19.

They swooped amid fears for the public's safety after Baryalei allegedly spoke by phone with Omarjan Azari on Tuesday to discuss a plan to kidnap and murder – presumably by beheading – a random person on camera, the footage of which would then be posted online by the Islamic State's sophisticated and powerful propaganda wing.

Australian Federal Police assistant commissioner Neil Gaughan told Fairfax Media that the terrorism alert level would have to stay on "high" despite the arrests – perhaps for a further 18 months.

"There is no doubt in my view the terror alert needs to stay high for the foreseeable future. We have seen an unprecedented increase in activity over the last 12 months," he said.

"And it's based on a lot of factors – ISIL, the declaration of the caliphate, an intent by people to do things in support of that, people continuing to recruit … We are in totally uncharted waters. I do not see any respite in the next 18 months."

Up to 160 Australians have either been involved in fighting in the Middle East or actively supporting it, officials have said.

At least 20 are believed to have returned to Australia and pose a security risk, the head of the national spy agency said last week.

Highlighting the risk of homegrown militants returning from the Middle East, Abbott pledged on Sunday to send a 600-strong force as well as strike aircraft to join a U.S.-led coalition fighting Islamic State militants in Iraq.